Control of bioreactors exhibiting two input multiplicities, i.e., stea
dy-state gains having opposite sign, is theoretically analyzed. The no
nlinear system is represented by a unity gain linear system cascaded w
ith a nonlinear gain. A conventional PI controller designed for the li
near portion of the system followed by the solution of the nonlinear g
ain equation gives a nonlinear controller. The performance of the nonl
inear controller is compared with that of the linear PI controller des
igned for the overall linear system. The nonlinear PI controller perfo
rmance is superior to that of the linear PI controller.